The present invention relates to subterranean fluids, and more particularly, to subterranean fluids comprising a hydraulic cement and an invert emulsion, and methods of using such fluids in subterranean operations.
During the drilling of a well bore in a subterranean formation, a drilling fluid may be circulated through a drill pipe and drill bit into the well bore, and subsequently flow upward through the well bore to the surface. The drilling fluid, inter alia, cools the drill bit, lubricates the rotating drill pipe to prevent it from sticking to the walls of the well bore, prevents blowouts by providing hydrostatic pressure to counteract the sudden entrance into the well bore of high pressure formation fluids, and removes drilled cuttings from the well bore. Typically, after a well bore is drilled to a desired final depth, the drill pipe and drill bit are withdrawn from the well bore and the drilling fluid is left therein so as to, inter alia, provide hydrostatic pressure on permeable formations penetrated by the well bore, thereby preventing the flow of formation fluids into the well bore.
A common subsequent step in completing the well bore usually involves placing a pipe string (e.g., casing), into the well bore. Depending upon factors such as, inter alia, the depth of the well bore and any difficulties in placing the pipe string therein, the drilling fluid may remain relatively static in the well bore for an extended period of time (e.g., up to about 2 weeks). While drilling fluids generally are not settable (e.g., they do not set into hard impermeable sealing masses when static), drilling fluids may increase in gel strength over time. Accordingly, the drilling fluid progressively may increase in gel strength, during the time in which it remains static, such that portions of the drilling fluid in the well bore may become increasingly difficult to displace.
Upon placement of the pipe string in the well bore, primary cementing typically is performed. This commonly involves cementing the pipe string disposed in the well bore by pumping a cement composition through the pipe string and into an annulus between the pipe string and the walls of the well bore, thereby displacing the drilling fluid in the annulus. However, if the drilling fluid has developed sufficient gel strength while in the well bore, an operator may be unable to displace all of the drilling fluid with the cement composition. Accordingly, the cement composition may bypass portions of the drilling fluid in the well bore. This may be problematic, inter alia, because the drilling fluid generally is not settable; therefore, formation fluids may enter and flow along the well bore, which is highly undesirable.
Operators have attempted to solve this problem by developing settable spotting fluid compositions, inter alia, to displace drilling fluids from well bores promptly after their use. However, these methods often have not met with success, as conventional settable spotting fluids commonly include blast furnace slag and other hydraulic components that may begin to set at relatively low temperatures, e.g., temperatures less than about 90° F. Also, certain slag-containing settable spotting fluids may be intolerant to cement composition contamination, causing the settable spotting fluids to set prematurely upon contact with well cement. Operators often attempt to counteract this tendency to prematurely set by adding a strong set retarder to the spotting fluid, and/or by separating the spotting fluid from the cement composition through the use of a spacer fluid.
Conventional settable spotting fluids also may demonstrate other problems, including, but not limited to, undesirable instability as well as a general inability to develop significant compressive strength upon setting. For example, the invert emulsions found in many conventional settable spotting fluids typically become unstable within about one week after their formulation. This is problematic, inter alia, because it often necessitates formulating the invert emulsion shortly before placing the settable spotting fluid in a subterranean formation. If an excessive amount of the invert emulsion is formed, it generally cannot be re-used and often is disposed of, which disposal may further increase the cost of a particular job. Furthermore, the general inability of most conventional settable spotting fluids to develop significant compressive strength upon setting also is problematic, because, inter alia, where formation fluids are present under a pressure sufficient to overcome the settable spotting fluid's low compressive strength, such formation fluids may continue to enter and flow along the well bore, which is undesirable and defeats a major purpose of using settable spotting fluids, which is to provide zonal isolation.